There are presently a number of systems available for controlling the generation of mist in nurseries in particular for plant propogation. In one system time control is used wherein spraying devices are switched on and off under the control of timers so that spray or mist may be applied to the plants at set intervals. Such systems are normally pre-set according to the ambient conditions and thus if the ambient conditions change the timers have to be adjusted so as to compensate therefor. A second system employs a balance arm principle to control the generation of mist. Such a system depends upon the weight difference between a wet and dry piece of gauze or similar material and a counterweight situated at the opposite ends of a lever arm. As long as the gauze is wet the counterweight will hold the lever in balance. If the gauze dries up, for example, when the relative humidity falls, the weight of the gauze will be reduced thus causing the lever arm to tilt and activate a switch to turn on the spraying systems. The moisture absorbed by the gauze as a result of the turning on of the system will add to the gauze weight thereby gradually restoring the state of equilibrium which will cause switching off of the sprayers. The disadvantage of this system, however, is that the state of equilibrium is very fragile and easily unbalanced by vibration--or air currents. Furthermore, a salt or algae buildup on the gauze will change the weight distribution of the system necessitating frequent cleaning or recalibration.
A further moisture control system is an artificial leaf sensor control which depends on electric currents conducted by water on an artificial leaf-like surface. If water is present between two probes on the surface the current will flow whilst the absence of a current will indicate a dry surface which absence can be used as a signal to activate the spraying equipment. Unfortunately, salts contained in water which transform the non-conductive water into a conductive liquid remain on the surface after the water itself has evaporated. When the water dries up some salt deposits are always left and they allow the current to flow even in the absence of water, thus rendering the control system unreliable. Furthermore, a buildup of grease on the surface will prevent a water film from being formed between the probes. The above shortcomings necessitate frequent cleaning to avoid malfunctions.